The shaped, green, beta-type alumina ceramic bodies to which the encapsulation chamber and sintering process of this invention are applicable are particularly useful as solid electrolytes in energy conversion devices, particularly those employing molten metals and/or molten metal salts as reactants.
All of these beta-type alumina ceramics exhibit a generic crystalline structure which is readily identifiable by X-ray diffraction. Thus, Beta-type alumina or sodium Beta-type-alumina is a material which may be thought of as a series of layers of aluminum oxide held apart by columns of linear Al-O bond chains with sodium ions occupying sites between the aforementioned layers and columns. Among the numerous polycrystalline Beta-type-alumina materials which may be processed using the encapsulation chamber and process of this invention are the following:
(1) Standard Beta-type-alumina which exhibits the above-discussed crystalline structure comprising a series of layers of aluminum oxide held apart by layers of linear Al-O bond chains with sodium occupying sites between the aforementioned layers and columns. Beta-type-alumina is formed from compositions comprising at least about 80% by weight, preferably at least about 85% by weight of aluminum oxide and between about 5 and about 15 weight percent, preferably between about 8 and 11 weight percent, of sodium oxide. There are two well-known crystalline forms of Beta-type-alumina, both of which demonstrate the generic Beta-type-alumina crystalline structure discussed hereinbefore and both of which can easily be identified by their own characteristic X-ray diffraction pattern. Beta-alumina if one crystalline form which may be represented by the formula Na.sub.2 0.11Al.sub.2 O.sub.3. The second crystalline form is .beta."-alumina which may be represented by the formula Na.sub.2 0.6Al.sub.2 O.sub.3. It will be noted that the .beta." crystalline form of Beta-type-alumina contains approximately twice as much soda (sodium oxide) per unit weight or material as does the Beta-alumina.
(2) Boron oxide B.sub.2 O.sub.3 modified Beta-type-alumina wherein about 0.1 to about 1 weight percent of boron oxide is added to the composition.
(3) Substituted Beta-type-alumina wherein the sodium ions of the composition are replaced in part or in whole with other position ions which are preferably metal ions.
(4) Beta-type-alumina which is modified by the addition of a minor proportion by weight of metal ions having a valence not greater than 2 such that the modified Beta-type-alumina composition comprises a major proportion by weight of ions of aluminum and oxygen and a minor proportion by weight of a metal ion in crystal lattice combination with cations which migrate in relation to the crystal lattice as result of an electric field, the preferred embodiment for use in such electrical conversion devices being wherein the metal ion having a valence not greater than 2 is either lithium or magnesium or a combination of lithium and magnesium. These metals may be included in the composition in the form of lithium oxide or magnesium oxide or mixtures thereof in amounts ranging from 0.1 to about 5 weight percent. A particularly advantageous method for manufacturing this type of modified or stabilized beta-alumina is disclosed in U.S. patent application Ser. No. 788,572 filed Apr. 18, 1977 now Pat. No. 4,113,928. That method, also called the "Zeta Process" results in a uniform distribution of the selected metal ions throughout the ceramic composition, thus allowing more complete conversion of .beta.-alumina to .beta."-alumina during sintering.
Many of the above polycrystalline materials and some of the electrical conversion devices in which they may be employed as a solid electrolytes are also disclosed in the following U.S. Pat. Nos.: 3,404,032; 3,404,036; 3,413,150; 3,446,677; 3,458,356; 3,468,709; 3,468,719; 3,475,220; 3,475,223; 3,475,225; 3,535,163; 3,719,531; and 3,811,943.
The bi- and multi-metal oxides having the beta-alumina type crystalline lattice make particularly effective separators and/or solid electrolytes for use in energy conversion devices. In the operation of such energy conversion devices, the cations such as sodium in the polycrystalline bi- or multi-metal oxide, or some other cation which has been substituted for sodium in part or whole, migrate in relation to the crystal lattice as a result of effects caused by an electric field. Thus, the solid ceramic electrolytes for which the sintering method of this invention is particularly suited provide selective cationic communication between the anodic and cathodic reaction zones of the energy conversion devices and are essentially impermeable to the fluid reactants employed in the device when the reactants are in the elemental, compound or anionic state. Among the energy conversion devices in which the particular sintered polycrystalline bior multi-metal oxides, e.g., tubes, enveloped, etc., are useful are: (1) primary batteries employing electrochemically reactive oxidants and reductants in contact with and on opposite sides of the solid electrolyte; (2) secondary batteries employing molten, electrochemically reversibly reactive oxidants and reductants in contact with and on opposite sides of the solid electrolyte; (3 ) thermoelectric generators wherein a temperature and pressure differential is maintained between anodic and cathodic reaction zones and/or between anode and cathode and a molten alkali metal is converted to ionic form, passed through the polycrystalline wall or inorganic membrane and reconverted to elemental form; and (4) thermally regenerated fuel cells.
The shaped ceramic bodies which are used as solid electrolytes in such energy conversion devices must be of uniform composition and high quality, e.g., good electrical characteristics. For many applications, particularly where tubes or rods are employed, it is also critical that the bodies be free of warpage and bending. The prior art teaches many methods of sintering polycrystalline materials, but in many cases the shaped sintered bodies are warped or bent to a greater or lesser degree. One explanation for such bending or warping of the shaped member is that temperature gradients exist along the length and width thereof, thus resulting in variations in sintering rate and mechanism which, in turn, results in bending and warping.
In many other cases, the composition of the body being sintered varies or deviates from that largely as a result of the loss of volatile components, such that properties, including electrical characteristics are impaired. Because of this problem of loss of volatile constituents such as sodium oxide or soda from compositions such as beta-type-alumina, it had been necessary in the past, when sintering shaped ceramic bodies for use in electrical conversion devices, to sinter the green body while it is embedded or packed in beta-alumina powder. For example, it had been common practice to sinter the shaped, green ceramic bodies in a crucible such as a platinum-rhodium crucible in which it is packed in coarse powder of beta-alumina, i.e., particles of one micron diameter. While this method is effective in maintaining the soda content of the beta-alumina ceramic, it is particularly troublesome in that it is difficult to remove the sintered shaped ceramic body from the surrounding sintered beta-alumina packing. This, of course, is time consuming and does not render the method acceptable for any type of commercial preparation of the ceramic member. Still another possible disadvantage of the process will be discussed hereinafter.
An alternative to sintering a body while it is packed in a powder of the same or similar composition comprises encapsulating or enclosing the green body to be sintered in a container formed from a noble metal which maintains its shape at the sintering temperature of the body. Noble metals which may be employed as the sintering tube or envelope in the process includes platinum, rhodium, alloys of two or more noble metals and alloys of noble metals with non-noble metals. This noble metal encapsulation process results in shaped bodies which are bend and warp free. To a certain extent it also overcomes the problem of loss of volatile metal oxide, e.g., sodium oxide, from the ware as it is sintered. However, some volatile loss still occurs with the process and the process is highly uneconomical.
A further alternative which might be considered would be saturating the atmosphere in the sintering frunace at the correct vapor pressure so that sintering can be conducted without encapsulation. Two problems exist with this approach, however, First, the Na.sub.2 O environment is very corrosive and would attack furnace heating elements such as MoSi.sub.2 (Super Kanthal) which are used frequently in oxidizing environments over 1500.degree. C. High Na.sub.2 O activities in the furnace atmosphere may also lead to enhanced corrosion of furnace refractories, setters and saggers. Both of these potential corrosion problems will lead to increased sintering costs in production. Second, tubes which are sintered without any encapsulation may be subject to severe thermal gradients during sintering and subsequent cooling which can lead to problems of warpage, thermal shock cracking, and the introduction of residual stresses in the ceramic after sintering.